Polyimides are known for their thermal-oxidative stability and XU 218, which is copolymer of benzophenonetetracarboxylic dianhydride (BTDA) and 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindane (DAPI) is one such polyimide available from CIBA-GEIGY Corporation. The composition of the copolymer is described in U.S. Pat. No. 3,856,752. Use of DAPI as an epoxy curing agent is described in U.S. Pat. No. 3,983,092.
Critical to the production of acceptable quality XU 218 is the purity of the two monomers, BTDA and DAPI, and the stoichiometric ratio of these two compounds. Prior to this invention, crystallization and purification of the DAPI monomer was not carried out, and without adequate purity, the XU 218 polyimide could not be reproducibly made. The DAPI was previously used as a melt in toluene which leads to severe handling problems, oxidation, potential health hazards and economic disadvantages due to off-standard polymer production. The full economic potential of the DAPI monomer can be realized through the crystallization and purification of the compound.
The preparation of these diamines is described in an article by J. C. Petropoulos et al., J. Amer. Chem. Soc., 80 1938 (1957), in U.S. Pat. Nos. 2,855,420 and particularly 3,983,092.
The diamines are obtained as a mixture of isomers as is described in U.S. Pat. No. 3,983,092 as a low melting glassy solid which may contain sufficient impurities to interfere at times with the subsequent polymerization step to form the polyimide.
A more highly purified diamine such as would be obtained in a crystallized form would be expected to yield the desired polyimide in a reproducible fashion.